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United States Patent |
5,158,262
|
Kamerbeek
,   et al.
|
October 27, 1992
|
Device for interrupting a material flow
Abstract
A device is set forth for interrupting a material flow by means of a
shutter 5 fastened on a spindle 11, which spindle 11 is rotatably
supported in a housing 1 of the device. The device is provided with a
torsion spring 33 which is elastically deformable through rotation of the
spindle 11, with substantially equal amounts of mechanical energy being
stored in the torsion spring 33 through elastic deformation in a first
position of the shutter 5. The material flow is shut off during this
operation, and in a second position of the shutter 5 the material flow
begins. The spindle 11 is provided with a permanent magnet 57 having pole
shoes 61, 63 which are held against stops 109, 115 of a magnetic yoke 99
in the first position under the influence of the magnetic field of the
magnet 57, and which are held against stops 111, 113 of the yoke 99 in the
second position. The yoke 99 is provided with an electric coil 121. The
shutter 5 is displaceable from a rest position to one of the two positions
by an alternating current in the coil 121, whereas a direct current in the
coil 121 attenuates the magnetic field of the magnet 57, so that the
shutter 5 is displaced from the relevant position to the other position
under the influence of the torsion spring 33. The device is suitable for
use in a system for the deposition of materials in the form of thin layers
on substrates. In such a system, the shutter 5 periodically interrupts a
material flow coming from a source of material and aimed at the substrate,
so that a desired composition of the layer is obtained.
Inventors:
|
Kamerbeek; Evert M. H. (Eindhoven, NL);
van der Borst; Albertus J. C. (Eindhoven, NL);
Larsen; Poul K. (Eindhoven, NL);
van der Leek; Johannes J. (Eindhoven, NL);
Meerman; Wilhelmus C. P. M. (Eindhoven, NL);
Van Stiphout; Nicolaas H. J. M. (Eindhoven, NL)
|
Assignee:
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U.S. Philips Corporation (New York, NY)
|
Appl. No.:
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691753 |
Filed:
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April 26, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
251/129.11; 251/65; 251/337 |
Intern'l Class: |
F16K 031/08 |
Field of Search: |
251/129.11,337,65
|
References Cited
U.S. Patent Documents
3484074 | Dec., 1969 | Lynes et al.
| |
3532121 | Jan., 1970 | Sturman.
| |
3974850 | Aug., 1976 | Pierson | 251/337.
|
4625943 | Dec., 1986 | Groger | 251/337.
|
4895344 | Jan., 1990 | Brand et al. | 251/129.
|
4913114 | Apr., 1990 | Kalippke et al. | 251/129.
|
4976237 | Dec., 1990 | Bollinger | 251/129.
|
Primary Examiner: Rosenthal; Arnold
Attorney, Agent or Firm: Miller; Paul R.
Claims
We claim:
1. A device for interrupting material flow comprising:
(a) a housing,
(b) a shutter being displaced between a first position and a second
position,
(c) mechanical spring means for moving said shutter between said first and
second position, said mechanical spring means including at least one
spring being elastically deformed by displacement of said shutter relative
to said housing, wherein substantially equal amounts of mechanical energy
are stored in said spring means during elastic deformation to said first
position and to said second position,
(d) permanent magnet means for holding said shutter relative to said
housing in each of said first position and in said second position against
a spring force of said spring means, said permanent magnet means being
fastened to a rotating spindle, and
(e) an electric field attenuation coil fastened to said housing, said
permanent magnet means resting against stops of a magnetic yoke of said
electric field attenuation coil in either said first position or in said
second position.
2. A device according to claim 1, wherein said permanent magnet means
includes pole shoes, said pole shoes resting against said stops.
3. A device according to claim 1, wherein said shutter is rotatably
supported in said housing, and wherein said spring means is a torsion
spring being fastened to said housing near a first end and being fastened
to said rotating spindle near a second end.
4. A device according to claim 3, wherein said torsion spring is provided
by a circular cylindrical bush having a helical incision.
5. A device according to claim 1, wherein an electric excitation coil is
disposed to displace said shutter from a rest position to either said
first position or to said second position, and wherein no mechanical
energy is stored in said spring means at said rest position.
6. A device according to claim 5, wherein said electric field attenuation
coil and said electric excitation coil are integrated.
Description
The invention relates to a device for interrupting a material flow by means
of a shutter which is displaceable relative to a housing of the device
from a first position to a second position, the material flow being shut
off during operation in one of the two positions.
BACKGROUND OF THE INVENTION
A device of the kind described above is known from the MBE-Review, no. 2,
March 1990, published by VG Semicon Limited. The known device is used in
systems for depositing materials, such as, for example, superconducting
materials, in the form of thin layers on substrates. In such systems, a
substrate is consecutively exposed to different material flows, each of
which is derived from a source of material which is active continuously
during a certain time. A predetermined thickness and composition of the
thin layers can be obtained since the material flows are each periodically
interrupted by a shutter. In the known device, the shutter is displaceable
by means of a pneumatic drive unit which is situated outside the
deposition chamber of the system, which is in a vacuum condition, in a
system as described above.
A disadvantage of the known device is that for each displacement of the
shutter the drive unit has to supply a quanity of energy required for the
displacement which is to be provided from the exterior. In addition,
unequal and inaccurate layer thicknesses are created in the deposition of
layers having a thickness of only a few atom diameters, for which very
short deposition times are used, because the time required for a
displacement of the known shutter is too great in relation to the
deposition time used owing to the mass inertia in the drive unit.
SUMMARY OF THE INVENTION
The invention has for its object to provide a device for interrupting a
material flow which does not exhibit the disadvantages mentioned above.
The device according to the invention is for this purpose characterized in
that the device comprises a mechanical spring assembly having at least one
mechanical spring which is elastically deformable by a displacement of the
shutter relative to the housing with substantially equal amounts of
mechanical energy being stored in the spring assembly through elastic
deformation in the first position and in the second position. Thanks to
the use of the spring assembly, a quantity of energy stored in the spring
assembly through elastic deformation during a previous displacement is
available for each subsequent displacement of the shutter, so that the
device requires only a comparatively small supply of energy. In addition,
a desired closing and opening time can be obtained through an optimization
of the spring stiffness of the mechanical spring and the moving mass of
the shutter.
A particular embodiment of a device according to the invention, which
provides a simple support for the shutter and a practical construction of
the spring assembly, is characterized in that the shutter is rotatably
supported in the housing, while the mechanical spring is a torsion spring
which is fastened to the housing near a first end and which is fastened to
a rotation spindle of the shutter near a second end.
A further embodiment of a device according to the invention, in which the
difference between the quantities of mechanical energy stored in the
spring assembly in the first position and in the second position is so
small as to be negligible, is characterized in that the torsion spring is
formed by a circular cylindrical bush having a helical incision.
A yet further embodiment of a device according to the invention, in which
no further drive unit is required in addition to the spring assembly, is
characterized in that the device is provided with means for holding the
shutter relative to the housing in each of the two positions against the
spring force of the spring assembly.
A special embodiment of a device according to the invention is
characterized in that the means comprise a permanent magnet, while the
device is also provided with an electric field attenuation coil. The use
of the permanent magnet means that the shutter can be held in either of
the two positions in a reliable manner and without energy being supplied.
The field attenuation coil generates an electromagnetic field during a
short period with a polarity opposite to that of the magnetic field of the
permanent magnet, so that the field of the permanent magnet is attenuated
and the shutter is moved to the other position under the influence of the
spring assembly.
A further embodiment of a device according to the invention, which provides
an accurate operation of the device thanks to a favourable magnetic field
characteristic, is characterized in that the magnet is fastened to the
rotation spindle and the field attenuation coil is fastened to the
housing, the magnet resting against stops of a magnetic yoke of the field
attenuation coil in either of the two positions.
A still further embodiment of a device according to the invention is
characterized in that the magnet is provided with pole shoes which rest
against the stops in either of the two positions. The use of the pole
shoes protects the magnet from mechanical peak loads which occur when the
shutter reaches either of the two positions during a displacement. In
addition, a particularly effective magnetic field is provided through the
use of the pole shoes.
A particular embodiment of a device according to the invention is
characterized in that the device comprises an electric excitation coil
with which the shutter is displaceable to one of the two positions from a
rest position in which no mechanical energy is stored in the spring
assembly. When the excitation coil is provided with an alternating current
having an electrical frequency which is substantially equal to a
mechanical natural frequency determined by the spring stiffness of the
mechanical spring and the moving mass of the shutter, the shutter is
brought from the rest position into one of the two positions by means of a
small supply of energy.
A further embodiment of a device according to the invention, which provides
a compact construction of the device, is characterized in that the device
comprises an integrated field attenuation and excitation coil.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
The invention is explained in more detail below with reference to the
drawing, in which
FIG. 1 shows a front view of a device according to the invention,
FIG. 2 shows the device of FIG. 1 partly in side elevation and partly in
cross-section taken on the line II--II in FIG. 1,
FIG. 3 is a cross-section of the device taken on the line III--III in FIG.
2, and
FIG. 4 shows a graph representing the relation between, on the one hand,
the mechanical and magnetic moment exerted on a shutter of the device of
FIG. 1 and, on the other hand, the position of the shutter relative to a
housing of the device.
DESCRIPTION OF THE INVENTION
The device illustrated in FIGS. 1 to 3 is provided with a metal housing 1
and a metal plate-shaped shutter 5 which is rotatable relative to the
housing 1 about an axis of rotation 3 and which is situated in a plane
transverse to the axis of rotation 3. The shutter 5 is, as is shown in
FIG. 2, fastened to a round mounting plate 9 of a metal spindle 11 by
means of screw connections 7, the metal spindle 11 having a centerline 13
which substantially coincides with the axis of rotation 3 and which is
rotatably supported in bearing bushes 19 and 21 of the housing 1 with
journals 15 and 17, respectively. The bearing bushes 19 and 21 are made of
a synthetic material and are provided in a first bearing support 23, which
is fastened to the housing 1 by means of screw connection 25, and in a
second, circular cylindrical bearing support 27, which is fastened to the
housing 1 by means of screw connections 29 and which has a centerline 31
which coincides substantially with the axis of rotation 3, respectively.
As is shown in FIG. 2, the device further comprises a metal torsion spring
33 which is formed by a circular cylindrical bush 35 having a centerline
37 and a regular helical incision 39. Such an incision is provided during
the manufacture of the torsion spring 33 by means of, for example, spark
erosion. The torsion spring 33 thus comprises a number of turns 41 of a
rectangular cross-section which are mutually separated by the incision 39.
As is shown in FIG. 2, the torsion spring 33 is so fastened to the
mounting plate 9 of the spindle 11 near a first end 43 by means of the
screw connections 7 that the centerline 37 of the torsion spring 33
concides substantially with the centerline 13 of the spindle 11. A second
end 45 of the torsion spring 33 is enclosed in a circular cylindrical
recess 47 of the housing 1, which surrounds the end 45 of the torsion
spring 33 without clearance, and the second end 45 is locked in the recess
47 by means of a number of screws 51 which are provided in a wall 49 of
the recess 47 and which bear on the end 45 of the torsion spring 33.
As FIGS. 1 to 3 further show, a permanent magnet 57 made of a permanent
magnetic material such as, for example, samarium-cobalt is provided on a
spindle portion 53 of the spindle 11 situated near the bearing bush 19 and
provided with screwthread near an end 55, which permanent magnet is
provided with a bore-hole 59 and extends relative to the spindle portion
53 in an x-direction which is indicated in FIG. 3, wherein the permanent
magnetic 57 is perpendicular to the axis of rotation 3, and is coupled to
the spindle portion 53. Pole shoes 61 and 63 made of a soft magnetic
material such as, for example, cobalt-iron are fastened to the permanent
magnet 57, which is magnetized in the x-direction, the pole shoes oppose
one another diametrically in the x-direction. As FIG. 2 further shows, the
permanent magnet 57 and the pole shoes 61, 63 are fastened to the spindle
portion 53 by means of bolts 65 and 67 which are provided in bore-holes 69
and 71 provided in the pole shoe 61, bore-holes 73 and 75 provided in the
permanent magnet 57, bore-holes 77 and 79 provided in the spindle portion
53, and bore-holes 81 and 83 provided in the pole shoe 63, respectively,
each of the bore-holes 81 and 83 being provided with screwthread, while
the bore-holes 69, 73, 77, 81 and the bore-holes 71, 75, 79, 83, are
arranged coaxially, respectively, in radial direction. The permanent
magnet 57 and the pole shoes 61, 63 are enclosed in an axial direction
between a first closing plate 85, which forms part of the spindle 11, and
a second closing plate 87, which is secured around the spindle portion 53
by means of a nut 89 provided around the end 55 of the spindle portion 53.
As is shown in FIG. 3, the pole shoes 61 and 63 are provided with side
surfaces 91, 93 and side surfaces 95, 97, respectively, each of the pairs
of side surfaces (91, 97) and (93, 95) being situated in a plane through
the axis of rotation 3, while the pairs (91, 97) and (93, 95) are mutually
symmetrically mirrored relative to a plane which goes through the axis of
rotation 3 and the x-axis.
As is shown in FIGS. 2 and 3, the housing 1 of the device is provided with
a magnetic yoke 99 made of a soft magnetic material such as, for example,
cobalt-iron and having a substantially circular cylindrical outer wall
101. The yoke 99 extends parallel to the axis of rotation 3, the outer
wall 101 concentrically surrounding the spindle portion 53 of the spindle
11. The yoke 99 further comprises a first inner wall 103 and a second
inner wall 105, each of which forms a boundary of an imaginary circular
cylinder 106 which concentrically surrounds the spindle portion 53, the
inner walls 103 and 105 being symmetrically mirrored relative to a plane
of symmetry 107 which goes through the axis of rotation 3 and is indicated
in FIGS. 1 and 3.
The yoke 99 is further provided with flat stops (109, 111) and (113, 115)
for the pole shoes 61 and 63, respectively, each of the pairs (109, 115)
and (111, 113) being situated in a plane through the axis of rotation 3,
and the pairs (109, 115) and (111, 113) are mutually symmetrically
mirrored relative to the plane of symmetry 107.
As is shown in FIG. 3, the yoke 99 comprises between the stops 109 and 111
and between the stops 113 and 115 a first attachment wall 117 and a second
attachment wall 119, respectively, for an electric coil 121, the
attachment walls 117 and 119 each forming a boundary of an imaginary
circular cylinder 123 which surrounds the spindle portion 53 substantially
concentrically. Recesses 125 extending parallel to the axis of rotation 3
are provided in each of the attachment walls 117, 119. Copper wire 127
provided with an electric insulation sheath is wound in the recesses 125
of the attachment walls 117 and 119, FIG. 1 indicating how portions 129 of
the copper wire windings provided in the attachment wall 117 are connected
near a front 131 of the yoke 99 to portions 133 of the copper wire
windings provided in the attachment wall 119. The portions 129 and 133 of
the copper wire windings are interconnected in a corresponding manner near
a rear side 135 of the yoke 99 indicated in FIG. 2.
The end 45 of the torsion spring 33 is so locked in the recess 47 by means
of the screws 51 that the torsion spring 33 exerts no mechanical moment on
the spindle 11 and on the shutter 5 mounted on the spindle 11 in a rest
position of the shutter 5 in which the x-axis is in the plane of symmetry
107. When the shutter 5 is rotated from the rest position about the axis
of rotation 3, the torsion spring 33 is elastically deformed and exerts a
mechanical moment M.sub.spring on the spindle 11 about the axis of
rotation 3. In the torsion spring 33 described above, the value of the
moment M.sub.spring is substantially proportional to the size of an angle
.phi. enclosed by the x-axis and the plane of symmetry 107 (see FIGS. 1
and 3), so that the quantity of mechanical energy stored through elastic
deformation in the torsion spring 33 is substantially the same in a first
position of the shutter 5, in which the side surfaces 93 and 95 of the
pole shoes 61 and 63 rest against the stops 111 and 113 of the yoke 99,
respectively, and in which .phi.=.phi..sub.max, and in a second position
of the shutter 5, in which the side surfaces 91 and 97 of the pole shoes
61 and 63 rest against the stops 109 and 115 of the yoke 99, respectively,
and in which .phi.=.phi..sub.min =-.phi..sub.max. The relation between
M.sub.spring and .phi. is shown in the graph of FIG. 4.
Besides the moment M.sub.spring of the torsion spring 33, a moment
M.sub.magn is exerted on the spindle 11 about the axis of rotation 3 by
the magnetic field of the permanent magnet 57, which has a direction
opposite to that of the moment M.sub.spring and which has a value which is
chiefly determined by the portions of the magnetic field directed towards
the stops 109, 111 and 113, 115 of the yoke 99 from the side surfaces 91,
93 and 95, 97 of the pole shoes 61 and 63, respectively. The moment
M.sub.magn is negligibly small for comparatively small values of the angle
.phi. because in that case the distance between the pole shoes 61, 63 and
the stops 109, 111, 113, 115 is great. The moment M.sub.magn has a maximum
value when the shutter 5 is in the first position or in the second
position, with the pole shoes 61, 63 resting against the stops 111, 113 or
the stops 109, 115, respectively. When the shutter 5 is rotated from the
first or the second position about the axis of rotation 3, the value of
the moment M.sub.magn decreases strongly owing to the increasing distance
between the pole shoes 61, 63 and the stops 111, 113 or the stops 109,
115, respectively. The relation between M.sub.magn and .phi. is shown in
the graph of FIG. 4.
The torsion spring 33, the permanent magnet 57, the pole shoes 61, 63 and
the yoke 99 are so dimensioned that the moment M.sub.magn is greater than
the moment M.sub.spring in the first and in the second position of the
shutter 5. It is achieved in this way that the pole shoes 61, 63 are held
against the stops 111, 113 and the stops 109, 115, respectively, against
the spring force of the torsion spring 33 in either of the two positions
under the influence of the magnetic field of the permanent magnet 57.
The device operates as follows. To start the device from the rest position,
in which the x-axis is situated in the plane of symmetry 107, the electric
coil 121 is provided with an alternating current having an electrical
frequency which is substantially equal to the mechanical natural frequency
of the device. To this end, the device is provided with an electronic
control unit 137 which is electrically connected to electrical connections
139 and 141 of the coil 121. The mechanical natural frequency of the
device is determined by the torsional rigidity of the torsion spring 33
and by the moment of mass inertia of the torsion spring 33, the spindle
11, the permanent magnet 57 and the pole shoes 61, 63, as well as by the
damping of the device. Owing to the interaction between the magnetic field
of the permanent magnet 57 and the magnetic field generated by the
alternating current in the coil 121, a moment M.sub.em is exerted on the
spindle 11 about the axis of rotation 3 with a frequency which is equal to
the electrical frequency of the alternating current and the mechanical
natural frequency of the device. The spindle 11 with the shutter 5
performs a rotational vibration with an increasing amplitude under the
influence of the moment M.sub.em until the pole shoes 61, 63 are held in
the first or in the second position under the influence of the magnetic
field of the permanent magnet 57. The coil 121 thus acts as an excitation
or starting coil. The device possesses only a small quantity of mechanical
damping, so that the shutter 5 is brought from the rest position into the
first or second position within a few cycles of the alternating current.
When used in a system for the deposition of materials in the form of thin
layers on a substrate, the shutter 5 shuts off a window 143 which is
diagrammatically depicted in FIG. 1 in the first position during
operation, so that a material flow, which comes from a continuously
operating source of material and which is aimed at the substrate, is
interrupted. To displace the shutter 5 from the first position to the
second position and expose the substrate to the material flow, the control
unit 137 passes a direct current through the coil 121 for a short period.
The strength and direction of the direct current are such that the
magnetic field of the permanent magnet 57 at the area of the pole shoes
61, 63 is attenuated by the electromagnetic field generated by the direct
current in the coil 121, so that the moment M.sub.spring of the torsion
spring 33 is greater than the sum of the moment M.sub.magn exerted by the
magnetic field and the moment M.sub.em exerted by the electromagnetic
field. As a result, the shutter 5 is displaced from the first to the
second position under the influence of the moment M.sub.spring, the pole
shoes 61, 63 being held against the stops 109, 115 by the permanent magnet
57 upon reaching the second position. Thus the coil 121 acts as a field
attenuation coil for the magnetic field of the permanent magnet 57. As was
stated above, the device posseses a small mechanical and electromagnetic
damping, caused by friction between the journals 15, 17 and the bearing
bushes 19, 21 and by eddy currents in the yoke 99, respectively. Depending
on the extent of the damping, a stronger direct current may be passed
through the coil 121 than is strictly speaking necessary for attenuating
the magnetic field of the permanent magnet 57. A quantity of kinetic
energy is thus transferred by the electromagnetic field of the coil 121 to
the shutter 5 which is necessary for overcoming the damping present during
the displacement of the shutter 5.
A displacement of the shutter 5 from the second to the first position takes
place in a corresponding manner. The torsion spring 33 forms an energy
buffer during this, which buffer supplies the quantity of kinetic energy
necessary for a displacement of the shutter 5 stored in the torsion spring
33 through elastic deformation during the preceding displacement of the
shutter 5. The device thus requires only a small supply of energy, since a
current through the coil 121 is necessary during a comparatively short
period only for the displacement of the shutter 5. No energy supply is
required for holding the shutter 5 in either of the two positions.
The time required by the shutter 5 for a displacement from the first to the
second position or a displacement from the second to the first position,
the so-called opening or closing time, respectively, is determined by the
mechanical natural frequency f.sub.res of the device and is substantially
equal to 1/(2.f.sub.res). The desired opening and closing times are
obtained through optimization of the moment of mass inertia of the
rotating parts of the device and the torsional rigidity of the torsion
spring 33. The device is notable in that a very short opening and closing
time can be obtained with it. Such opening and closing times are desirable
in the position of layers having a thickness of only a few atom diameters,
for which short deposition times are used. It is necessary for the opening
and closing times of the shutter 5 to be short in relation to the
deposition timed used in order to obtain an even thickness of the layers.
It should be noted that the device according to the invention has a compact
construction and a log, maintainance-free life, so that the device, also
in view of its small energy supply, is eminently suitable for application
in a vacuum chamber, such as, for example, the deposition chamber of a
system for the deposition of superconducting materials, which chamber is
under vacuum. Furthermore, the device may be used, for example, in systems
for semiconductor epitaxy, systems for deposition and epitaxy of metallic
multilayers and system for deposition and epitaxy of oxidic materials
which have a layer structure with specific crystallographic orientations.
It should further be noted that, instead of a rotary shutter, a translatory
shutter may also be used, for which, for example, conventional tension
springs or leaf springs may be used. The use of a rotary shutter, however,
provides the shutter with a simple support.
It is furthermore noted that the term torsion spring denotes a spring which
exerts a torque on a spindle when this spindle is rotated about the
centerline of the spindle. In the case of the torsion spring 33 used here
on which the shutter 5 is mounted near an end of the torsion spring, the
shutter 5 may also be fastened to the torsion spring near a central
surface of the torsion spring which is transverse to the centerline 37.
Instead of the torsion spring used here, a different type of torsion
spring may also be used, such as, for example, a torsion spring having
helical turns which are situated in one plane. An assembly of conventional
tension and compression springs may also be used instead of the torsion
spring 33. It is difficult, however, to set the rest position with
alternative spring types and other spring types, such as, for example, gas
springs.
It should further be noted that, instead of the electromagnetic means for
starting the shutter 5 and holding the shutter 5 in the two positions,
other means may alternatively be used. For example, separate permanent
magnets with field attenuation coils may be used for holding the shutter 8
in each of the two positions, and a separate permanent magnet having an
excitation coil may be used for starting the shutter 5. This, however,
leads to a more complicated construction of the device. In addition,
mechanical or electromechanical retention means may be used instead of a
permanent magnet with a field attenuation coil. These alternative
retention means, however, are less reliable in general, have a shorter
life, or require a greater supply of energy. The application range is more
restricted then. Furthermore, the means for holding the shutter 5 in the
two positions may be omitted if the device is provided with a drive unit
for the shutter 5, with which the shutter 5 can also be retained in the
two positions. The torsion spring then acts as an energy buffer with which
a reduction of the required energy supply of the drive unit is achieved.
Finally, it is noted that the permanent magnet 57 is protected against
mechanical peak loads by the pole shoes 61, 63, which loads occur when the
pole shoes 61, 63 hit against the stops 109, 115 or 111, 113. A yoke
without stops may alternatively be used instead of the magnetic yoke 99.
In general, however, a yoke without stops leads to a less accurate
operation of the device. This is because the use of the stops leads to a
favourable characteristic of the moment M.sub.magn as a function of the
angle .phi., the moment M.sub.magn being effective only if the pole shoes
61, 63 are close to the stops 109, 115 or 111, 113 (see FIG. 4). As a
result, the direct current needs to be supplied during a very short time
only (low power dissipation) and the total moment M.sub.spring +M.sub.magn
has a linear relation to the angle .phi. over a wide range (opening and
closing times can be accurately defined). Moreover, the use of the stops
achieves that the material deposited on the shutter 5 during operation and
causing an undesirable increase in the moment of mass inertia is flung off
the shutter 5 the moment the shutter 5 reaches either of the two positions
.
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